1. Field of the Invention
The present invention relates to a filtering device for filtering water of a kind used in, for example, pools and spas as it is recirculated or filtering water of a kind used in, for example, in a desalination plant.
2. Description of Related Art
In general, a sand filtering device is employed in circulatory filtration of water used in, for example, a bathtub or a pool. The sand filtering device is of such a structure which makes use of a gravel layer and a sand layer deposited thereon, through both of which water is allowed to pass so that finely divided particles suspended in the water can be removed. It has, however, been found that the device itself is bulky in size, thus involving a problem that a relatively large space for installation is required.
Also, other than the sand filtering device, a membrane filtering device is known, in which a membrane made of a synthetic resin of a kind capable of trapping finely divided particles is employed. As compared with the sand filtering device, the membrane filtering device is relatively small in size and can, therefore, be installed in a relatively small space. See, for example, the Patent Document 1 listed below.
In a seawater desalination plant, the ultrafiltration membrane or the reverse osmosis membrane or the like is employed, but the sand filtering device is often used in the front stage of the ultrafiltration membrane or reverse osmosis membrane.